The mechanical properties of of austenitic stainless steels are thought to be improved by the phenomenon of deformation-induced martensitic transformation, which acts to stabilise plastic flow and improve toughness. This is particularly the case at cryogenic temperatures where the martensite volume fractions increase. The examination of this phenomenon at cryogenic temperatures is therefore important not only for cryogenic applications but for fundamental understanding and modelling of industrially relevant steels. It has only recently become possible to examine in situ the evolution of microstructure during cryogenic mechanical testing, using the ENGIN-X stress rig cryocooler. This experiment will map the transformations as functions of temperature and strain and provide improved understanding on the interactions between the two martensite phases produced during cryogenic straining.